1. Field of the Invention
The present invention relates to a resin composition containing some inorganic particulate components with different mean diameters, an article formed from such a resin composition, and a mirror having such a formed article and a reflective layer.
2. Description of the Related Art
Some devices used in a precision optical system have organic resin components as the optical elements or surrounding components thereof. These resin-made components can cause the optical system to be misaligned when the size thereof is greatly influenced by the temperature. A known way to reduce such temperature-dependent changes in the size of a resin component is to use an organic resin material with a smaller coefficient of linear expansion.
When an organic resin material is used to make a mirror or a similar optical element, it is required to ensure the smoothness of an optically effective surface (i.e., a surface available for a certain optical application) of the optical element in addition to controlling the coefficient of linear expansion of the resin material.
Japanese Patent No. 4787549 discloses a method in which inorganic particles with a small mean diameter are added to an organic resin to reduce the coefficient of linear expansion of the organic resin material while ensuring the smoothness of the optically effective surface of articles formed from that material. The publication states that in this method inorganic particles having a mean diameter of 18 nm or less are added to an organic resin to produce a resin material with a reduced coefficient of linear expansion, the resulting resin material is injected into a mold into which a heated gas stream has been introduced, and a formed article is obtained with a coefficient of linear expansion of 18×10−6/° C. or less.
Japanese Patent Laid-Open No. 2006-36915 discloses a resin composition that can be used to seal electronic components. This resin composition contains two kinds of silica particles with different mean diameters.
Adding particles with a small diameter to a thermoplastic resin as a matrix, however, increases the melt viscosity of the resulting material. An increased melt viscosity causes some problems such as poor formability of the material in injection molding and other molding processes. The resin material used in the method disclosed in Japanese Patent No. 4787549 therefore has insufficient formability.
The resin composition disclosed in Japanese Patent Laid-Open No. 2006-36915 also lacks sufficient formability because of the high inorganic particle content and the consequent high melt viscosity. Furthermore, articles formed from this resin composition are unsatisfactory in terms of surface smoothness.